Subjects will be instructed to walk along a 17m walkway in which two force plates are embedded. The subjects will walk at their self-selected pace and practice trials will be performed until the subject could contact the force platforms with only one foot (not targeting) and with a constant velocity within 5% deviation. Speed will be measured by photoelectric beams located 3.06m apart, midway along the walkway. Experienced physical therapists will be responsible for patient information, instruction and data collection.